This application relates to a device for adapting the length of a pair of legs to the unevenness of a supporting surface. More particularly, the invention relates to a device which may be part of a ladder or adapted to be attached thereto for the purpose of permitting safe use of the ladder on uneven ground.
In Applicant's prior U.S. Pat. No. 3,794,141, the disclosure of which is incorporated herein by reference, there is disclosed a device which may be attached to a ladder so as to adjust the length of a pair of legs to the unevenness of the ground surface. Applicant's prior patent discloses a downwardly facing substantially U-shaped member adapted to be attached to the legs of a ladder. Reciprocally extending leg members project upwardly into the U-shaped tubular member and each extendable leg is joined to a continuous ball chain member which is guidably supported within the tubular member. The ball chain member comprises a central flexible core on which are threaded a plurality of balls suitably arranged so that upward force on one of the extendable legs causes the other extendable leg to be forced downward. When equal, upward force is applied on both of the extendable legs, the extendable legs and hence the ladder to which the device is attached is maintained in that orientation.
Applicant's prior patent also discloses and claims locking means attached to the tubular member, the locking means having an integrally formed, arcuate chamber positioned above the tubular member and having downwardly projecting pins within the arcuate chamber. When equal upward force is applied to both extendable legs, the ball chain member is forced up into the arcuate chamber such that the downwardly projecting pins are seated or positioned between the balls to prevent further reciprocal movement of the ball chain connecting member.
Subsequent to issuance of Applicant's prior U.S. patent, other leg levelling devices have been disclosed. For example, Canadian Pat. No. 992,510 which issued July 6, 1976 to Beguin provides a leg type support with automatic adjustment and means to lock an interconnected supporting element in place.
U.S. Pat. No. 4,497,390 issued Feb. 5, 1985 to Wilson also discloses a self levelling device for a ladder comprising an interconnecting tubular member in which a plurality of balls are guidably retained. The balls in the Wilson patent are not connected and, hence, are subject to longitudinal displacement if, for example, uneven forces are applied on the legs of the ladder.
U.S. Pat. No. 4,770,275 which issued Sept. 13, 1988 to Williams also discloses a leveller for a ladder comprising a pair of sliding legs telescopingly engagable in upright tubes. A connecting element in the form of a continuous flexible ligament such as a wire rope acts on the slidable legs so that as one leg is shortened the opposite leg is lengthened until equal upward force is imposed on both legs.
The deficiency of the prior art including Applicant's own prior patent involves the lack of positive locking of the legs in the position in which they accommodate an uneven surface. In all of the prior art devices known to Applicant, equal upward force must be applied on both extendable legs in order for the legs to remain in the proper position.
Therefore, there is a requirement for a device which may be attached to a ladder which accommodates uneven supporting surface wherein the legs are positively locked in the selected position. For example, if a ladder is being used on an uneven surface and the ladder is resting against a smooth, shiny surface such as a wall having aluminum siding or an eavestrough, and the person using the ladder reaches outwardly to one side of the ladder unequal forces will be applied to the legs. If the person using the ladder reaches still further outwardly to one side, all of the weight will be borne by one of the legs and a situation may exist where the opposite leg will lift off of the supporting surface. In the devices of the prior art, when the weight is removed from the opposite leg, the interconnecting ball chain or equivalent connecting member will permit the leg which does not bear the weight to extend further out of the supporting tube which, of course, means that the leg which is bearing all of the weight will extend further upwardly into the supporting tube. This will further compound the unequal weight situation and the ladder will quickly tip sidewardly thereby placing the person using the ladder in great danger. If, for example, the ladder is being used on a scaffold far above the ground, the person using the ladder may well fall to their death as a result of the shifting load on the legs of the ladder.